1. Field of the Invention
The invention relates generally to enthalpy exchangers and more particularly to an enthalpy exchanger comprising a heat-conducting wall along both sides of which two respective media can flow whereby heat may be transferred from one medium to the other through the wall.
2. Description of the Related Art
Enthalpy exchangers are known in which the heat-conducting wall is provided with a cover layer. This cover layer can fulfill one or more of the following functions: protecting against corrosion and/or erosion of each relevant surface; improvement of the thermal resistance; improvement of the appearance; improvement of impact resistance, for instance susceptibility to degradation as a result of loose chippings; improvement of the chemical and/or biochemical resistance of each relevant surface; changing frictional resistance and/or flow resistance; moisture absorption and/or evaporation.
Japanese Patent No JP63 319020 discloses a hygroscopic element having plural air penetrating passages. The element is dipped in a slurry consisting of a hygroscopic salt, silica gel, cement and other components. The resulting product is said to have excellent hygroscopic characteristics.
Another enthalpy exchanger described in U.S. Pat. No. 6,228,506 consists of a cellulose based substrate impregnated with a polymeric desiccant. A further vapor extraction apparatus is known from U.S. Pat. No. 5,565,139, which includes a gel sorbent capable of absorbing vapor directly into the liquid state. Prior art devices using hygroscopic materials have been found to exhibit excessive affinity to the absorbed water. Particularly for use in evaporative cooling devices, such affinity to water has been found to reduce the ability of the cooling device to operate efficiently. Furthermore, enthalpy exchangers produced by dipping into a slurry have been found to function ineffectively under certain conditions since the coating is applied non-selectively to all surfaces.
Use has also been made of other materials for the cover layer. Such materials include organic materials, inorganic materials, synthetic materials in the form of monomers and/or polymers, ceramic materials, metallic materials and hybrid materials. In the prior art the morphology of the cover layers, i.e. the internal microstructure and macrostructure may be continuous or discrete (particles); it may include a binder; it may be formed with capillary interstitial spaces for absorbing water; it may be hygroscopic, hydrophilic, hydrophobic; of varying thicknesses; or be formed of woven or non-woven material or filamentary materials.
Prior art cover layers have been found to exhibit various undesirable and uncontrollable side effects. These include in particular: an uncontrolled increase in thermal resistance, which is undesirable with respect to the necessary heat transfer; vulnerability to erosion and corrosion; susceptibility to diverse forms of degradation; inadequate adhesion, whereby the lifespan of the product is reduced; susceptibility to growth of micro-organisms; and susceptibility to build-up of dirt. In addition, existing layers, even with a limited controllability of desired properties, can only be applied by relatively costly production processes.
There is thus a need for a heat exchanger having a heat-conducting wall provided with a cover layer having controllable and homogeneous properties.
There is furthermore a particular need for a heat exchanger having a heat-conducting wall that is able to retain moisture in a cover layer of the heat-conducting wall and yet is able to operate efficiently as an evaporative cooler with relatively low resistance to heat-conduction across the cover layer.